1. Field of the Invention
The present invention relates to an automatic welding machine in which weld line or seam tracking is performed by measuring the distribution of a magnetic field leaking out of a weld line gap between objects to be welded.
2. Description of the Prior Art
Conventionally, an automatic welding machine is known in which a welding torch, while advancing along a weld line, is automatically driven in a direction perpendicular to the weld line to detect and track it. For example, laid-open Japanese Patent application No. 114452/77 discloses a method in which magnetic flux generated by a welding arc and leaking from the weld line gap is detected to identify the weld line. This application also discloses an automatic welding machine which employs a method in which thermal residual magnetism induced in welding one surface is detected in welding the other surface to thereby identify the weld line.
FIG. 1 shows an automatic welding machine which employs the above-mentioned prior art method, in which a truck 2 is movable on a rail 1 in the X direction. A torch drive mechanism 4 is provided on the side of the truck, and a welding torch 3 is supported at its forward end to be movable in each of the X, Y, and Z directions. Objects 5 to be welded are placed opposite the torch such that a gap 10 defining a weld line 9 is parallel to the rail 1.
In order to weld while following the gap 10, a sensor mount 7 is disposed adjacent the objects to be welded and a pair of magnetic sensors 6a and 6b for detecting magnetic flux leaking from the gap are respectively mounted at opposite sides of the gap on the sensor mount 7. The latter is supported at the end of a sensor drive mechanism 8 provided on the side of the truck 2. The mechanism 8 is responsive to the magnetic leakage flux detected by the respective sensors 6a and 6b to drive them in a direction perpendicular to the welding direction to seek the position of maximum leakage corresponding to the position of the weld line. Movements of the sensors attendantly control the movement of the torch 3, as is conventional.
During welding magnetic flux is induced by the current flowing from the torch 3 to the objects 5, and the density of this flux in the gap 10 is high when the objects are of a magnetic material. FIG. 2 shows the magnetic flux density distribution along the Y axis; as is apparent the flux density is maximum at the central portion C of the weld line gap 10, whereby automatic tracking can be performed by detecting the position of such maximum flux density. For example, the respective output signals of the sensors 6a and 6b may be fed to a comparator whose output in turn drives the sensor mount 7 in a direction to zero any difference between the sensor outputs. The X-Y coordinates of the detected maximum flux density position are stored as positional information so that welding can be performed by driving the torch 3 via the mechanism 4 to cause the torch to follow the weld line 9.
Such a conventional machine has disadvantages, however, in that since the mount 7, the sensors 6a and 6b, and the drive mechanism 8 are disposed separately from and in front of the welding torch 3 in the direction of travel, the structure is complicated and its ability to accurately track the weld line is limited. The conventional machine has a further disadvantage in that the driving of the sensors to bring their mid-point to the position of maximum flux density is always done at a constant speed regardless of the lateral error sensed, and it is thus impossible to smoothly stop the sensors at the desired position. The sensor mount 7 may thus hunt or oscillate until it is settled, resulting in poor tracking accuracy.